A buckle tensioner moves the buckle head in the case of retraint by approximately 50 to 100 mm towards the anchorage point of the buckle tensioner, in order to remove the belt slack from the safety belt system. The tensioning process itself takes place abruptly because generally pyrotechnic drive units are provided. Parts hindering the tensioning process, for example parts transported in the vehicle, must in no way protrude into the intermediate space between the buckle head and the drive device because the buckle head would strike against this part and it could not be displaced through the entire tightening distance. In addition, a risk of injury to occupants of the vehicle exists in known buckle tensioners for the instance which in fact scarcely occurs in practice but is theoretically not ruled out, that during the tensioning process, fingers of the occupant of the vehicle protrude into the intermediate space between buckle head and drive device, which would then be jammed.